Scanner We have written programs that print console output. It is - - PDF document

1

• Scanner

Readings: 3.4

• We have written programs that print console output.

It is also possible to read input from the console.

The user types the input into the console. The program uses the input to do something. Such a program is called an interactive program.

• Interactive programs can be challenging.

Computers and users think in very different ways. Users tend to “misbehave”.

• System.in

System.out is an object!

It has the methods named println and print for

printing to the console.

We read input using an object named System.in

System.in is not intended to be used directly. We will use another object, from a class called Scanner,

to read input from System.in.

• Scanner

Constructing a Scanner object to read the console:

Scanner <name> = new Scanner(System.in);

Example:

Scanner console = new Scanner(System.in);

• Scanner

Some methods of Scanner: Each of these methods pauses your program until

the user types input and presses Enter.

Then the value typed is returned to your program.

reads and returns user input as a double nextDouble() reads and returns user input as an int nextInt() reads and returns user input as a String next() Description Method

2

• Scanner

Example:

System.out.print("How old are you? "); // prompt int age = console.nextInt(); System.out.println("You'll be 40 in " + (40 - age) + " years.");

prompt: A message printed to the user, telling them

what input to type.

• token: A unit of user input, as read by the Scanner.
• Tokens are separated by whitespace (spaces, tabs, new lines).
• How many tokens appear on the following line of input?

23 John Smith 42.0 "Hello world"

• When the token doesn't match the type the Scanner tries to read,

the program crashes.

• Example:

System.out.print("What is your age? "); int age = console.nextInt(); Sample Run: What is your age? Timmy InputMismatchException: at java.util.Scanner.throwFor(Unknown Source) at java.util.Scanner.next(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) ...

• Java class libraries: A large set of Java classes

available for you to use.

Classes are grouped into packages. To use the classes from a package, you must include an

import declaration at the top of your program.

Import declaration, general syntax:

import <package name>.*;

Scanner is in a package named java.util

import java.util.*;

!

"

import java.util.*; // so that I can use Scanner public class ReadSomeInput { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("What is your first name? "); String name = console.next(); System.out.print("And how old are you? "); int age = console.nextInt(); System.out.println(name + " is " + age + ". That's quite old!"); } }

Sample Run:

What is your first name? Marty How old are you? 12 Marty is 12. That's quite old!

• "

import java.util.*; // so that I can use Scanner public class Average { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); double average = (num1 + num2 + num3) / 3.0; System.out.println("The average is " + average); } }

Sample Run:

Please type three numbers: 8 6 13 The average is 9.0

• Notice that the Scanner can read multiple values from one line.
• #

The main method in the previous program could be better

structured by grouping the collection of numbers into a method.

import java.util.*; // so that I can use Scanner public class Average { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); double average = (num1 + num2 + num3) / 3.0; System.out.println("The average is " + average); } }

3

• #

To have multiple methods read user input, declare a

Scanner in main and pass it to each method as a parameter.

public static void main(String[] args) { Scanner console = new Scanner(System.in); int sum = readSum3(console); double average = sum / 3.0; System.out.println("The average is " + average); } public static int readSum3(Scanner console) { System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); return num1 + num2 + num3; }

• " $%
• Consider changing the output to include the minimum value:

Please type three numbers: 8 6 13 The average is 9.0 The minimum value is 6

• How would we change the previous program?

public static void main(String[] args) { Scanner console = new Scanner(System.in); int sum = readSum3(console); double average = sum / 3.0; System.out.println("The average is " + average); // What goes here? } public static int readSum3(Scanner console) { System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); return num1 + num2 + num3; }

• & '

import java.util.*; // so that I can use Scanner public class Average { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); double average = (num1 + num2 + num3) / 3.0; System.out.println("The average is " + average); System.out.println("The minimum value is " + Math.min(num1, Math.min(num2, num3))); } }

• ()$*&

A person's body mass index (BMI) is computed as follows: Write a program that produces the following output: This program reads in data for two people and computes their body mass index (BMI) and weight status. Enter next person's information: height (in inches)? 62.5 weight (in pounds)? 130.5 Enter next person's information: height (in inches)? 58.5 weight (in pounds)? 90 Person #1 body mass index = 23.485824 Person #2 body mass index = 18.487836949375414 Difference = 4.997987050624587

703

2 ×

= height weight BMI

• # $*&

// This program computes two people's body mass index (BMI) // and compares them. The code uses parameters and returns. import java.util.*; // so that I can use Scanner public class BMI { public static void main(String[] args) { introduction(); Scanner console = new Scanner(System.in); double bmi1 = processPerson(console); double bmi2 = processPerson(console);

• utputResults(bmi1, bmi2);

} // prints a welcome message explaining the program public static void introduction() { System.out.println("This program reads in data for two people"); System.out.println("and computes their body mass index (BMI)"); System.out.println("and weight status."); System.out.println(); } // report overall results public static void outputResults(double bmi1, double bmi2) { System.out.println("Person #1 body mass index = " + bmi1); System.out.println("Person #2 body mass index = " + bmi2); double difference = Math.abs(bmi1 - bmi2); System.out.println("Difference = " + difference); }

• # $*&

// reads information for one person, computes their BMI, and returns it public static double processPerson(Scanner console) { System.out.println("Enter next person's information:"); System.out.print("height (in inches)? "); double height = console.nextDouble(); System.out.print("weight (in pounds)? "); double weight = console.nextDouble(); System.out.println(); double bmi = getBMI(height, weight); return bmi; } // Computes a person's body mass index based on their height and weight // and returns the BMI as its result. public static double getBMI(double height, double weight) { double bmi = weight / (height * height) * 703; return bmi; } }

4

• +,

Readings: 4.1

!

+,

Cumulative sum Fencepost loops

• "-

Consider the following code: Scanner console = new Scanner(System.in); System.out.print("Type a number: "); int num1 = console.nextInt(); System.out.print("Type a number: "); int num2 = console.nextInt(); System.out.print("Type a number: "); int num3 = console.nextInt(); int sum = num1 + num2 + num3; System.out.println("The sum is " + sum); Any ideas to improve the code?

• .

The variables num1, num2, and num3 are unnecessary:

Scanner console = new Scanner(System.in); System.out.print("Type a number: "); int sum = console.nextInt(); System.out.print("Type a number: "); sum += console.nextInt(); System.out.print("Type a number: "); sum += console.nextInt(); System.out.println("The sum is " + sum);

cumulative sum: A variable that keeps a sum-in-progress and is

updated many times until the task of summing is finished.

• The variable sum in the above code represents a cumulative sum.
• .

How could we modify the code to sum 100 numbers?

Creating 100 copies of the same code would be redundant.

An incorrect solution: Scanner console = new Scanner(System.in); for (int i = 1; i <= 100; i++) { int sum = 0; System.out.print("Type a number: "); sum += console.nextInt(); } System.out.println("The sum is " + sum); How could we modify the code to sum 100 numbers?

Creating 100 copies of the same code would be redundant.

An incorrect solution: Scanner console = new Scanner(System.in); for (int i = 1; i <= 100; i++) { int sum = 0; System.out.print("Type a number: "); sum += console.nextInt(); } System.out.println("The sum is " + sum); // sum out of scope

• .

A correct version:

Scanner console = new Scanner(System.in); int sum = 0; for (int i = 1; i <= 100; i++) { System.out.print("Type a number: "); sum += console.nextInt(); } System.out.println("The sum is " + sum);

Key idea: Cumulative sum variables must always be

declared outside the loops that update them, so that they will continue to live after the loop is finished.

5

• /
• The user's input can control the number of times the loop repeats:

Scanner console = new Scanner(System.in); System.out.print("How many numbers to add? "); int count = console.nextInt(); int sum = 0; for (int i = 1; i <= count; i++) { System.out.print("Type a number: "); sum += console.nextInt(); } System.out.println("The sum is " + sum);

Sample Run:

How many numbers to add? 3 Type a number: 2 Type a number: 6 Type a number: 3 The sum is 11

• . $()
• Write a program that reads input of the number of hours two

employees have worked and displays each employee's total and the

• verall total hours.
• The company doesn't pay overtime, so cap any day at 8 hours.

Sample Run:

Employee 1: How many days? 3 Hours? 6 Hours? 12 Hours? 5 Employee 1's total paid hours = 19 Employee 2: How many days? 2 Hours? 11 Hours? 6 Employee 2's total paid hours = 14 Total paid hours for both employees = 33

• . $#

// Computes the total paid hours worked by two employees. // The company does not pay for more than 8 hours per day. // Uses a "cumulative sum" loop to compute the total hours. import java.util.*; public class Hours { public static void main(String[] args) { Scanner input = new Scanner(System.in); int hours1 = processEmployee(input, 1); int hours2 = processEmployee(input, 2); int total = hours1 + hours2; System.out.println("Total paid hours for both employees = " + total); } ...

• . $#

... // Reads hours information about one employee with the given number. // Returns the total hours worked by the employee. public static int processEmployee(Scanner console, int number) { System.out.print("Employee " + number + ": How many days? "); int days = console.nextInt(); // totalHours is a cumulative sum of all days' hours worked. int totalHours = 0; for (int i = 1; i <= days; i++) { System.out.print("Hours? "); int hours = console.nextInt(); hours = Math.min(hours, 8); // cap at 8 hours per day totalHours += hours; } System.out.println("Employee " + number + "'s total paid hours = " + totalHours); System.out.println(); return totalHours; } }

• 123- 456

Readings: 4.1

!

74

Problem: Write a static method named printNumbers that

prints each number from 1 to a given maximum, separated by commas.

Example:

printNumbers(5) should print: 1, 2, 3, 4, 5

6

• " 5

public static void printNumbers(int max) { for (int i = 1; i <= max; i++) { System.out.print(i + ", "); } System.out.println(); // to end the line }

Output from printNumbers(5):

1, 2, 3, 4, 5,

• 235

public static void printNumbers(int max) { for (int i = 1; i <= max; i++) { System.out.print(", " + i); } System.out.println(); // to end the line }

Output from printNumbers(5):

, 1, 2, 3, 4, 5

• 74

We want to print n numbers but need only n - 1 commas. Similar to the task of building a fence

If we repeatedly place a post and wire, the last post has an extra

dangling wire.

A flawed algorithm:

for (length of fence) { plant a post. attach some wire. }

• The solution is to add an extra statement outside the loop

that places the initial "post."

This is called a fencepost loop. The revised algorithm:

plant a post. for (length of fence - 1) { attach some wire. plant a post. }

• 74

public static void printNumbers(int max) { System.out.print(1); for (int i = 2; i <= max; i++) { System.out.print(", " + i); } System.out.println(); // to end the line }

Output from printNumbers(5):

1, 2, 3, 4, 5

• 0 $()

Write a program that reads a base and a maximum power

and prints all of the powers of the given base up to that max, separated by commas.

Base: 2 Max exponent: 9 The first 9 powers of 2 are: 2, 4, 8, 16, 32, 64, 128, 256, 512

7

• if8else

Readings: 4.2

• .

“If you eat your vegetables, then you can

have dessert.”

“If you do your homework, then you may

go outside to play, or else you’ll be grounded for life.”

• 7if

if statement: A control structure that executes a block of

statements only if a certain condition is true.

General syntax:

if (<test>) { <statement(s)> ; }

Example:

double gpa = console.nextDouble(); if (gpa >= 3.0) { System.out.println("Good job! Have a cookie."); }

!

if 4 3

• 7if8else
• if/else statement: A control structure that executes one block of

statements if a certain condition is true, and a second block of statements if it is false. We refer to each block as a branch.

• General syntax:

if (<test>) { <statement(s)> ; } else { <statement(s)> ; }

• Example:

double gpa = console.nextDouble(); if (gpa >= 3.0) { System.out.println("Good job! Have a cookie."); } else { System.out.println("No cookie for you!"); }

• if8else 4 3

8

• 7/)

There is no such thing as an “if loop”—there is no loop!

• 9 )

The <test> used in an if or if/else statement is the

same kind seen in a for loop.

for (int i = 1; i <= 10; i++) {

These tests are called relational expressions and use the

following relational operators:

true 5.0 >= 5.0 greater than or equal to >= false 126 <= 100 less than or equal to <= true 10 > 5 greater than > false 10 < 5 less than < true 3.2 != 2.5 does not equal != true 1 + 1 == 2 equals == Value Example Meaning Operator

• ( )

Relational operators have lower precedence than math

• perators.

5 * 7 >= 3 + 5 * (7 - 1) 5 * 7 >= 3 + 5 * 6 35 >= 3 + 30 35 >= 33 true

Relational operators cannot be "chained" as they can in

algebra.

2 <= x <= 10 true <= 10 error!

• if8else$()

Write code to read a number from the user and print

whether it is even or odd using an if/else statement.

Example executions: Type a number: 42 Your number is even Type a number: 17 Your number is odd

• +3if8else

Loops can be used with if/else statements. int nonnegatives = 0, negatives = 0; for (int i = 1; i <= 10; i++) { int next = console.nextInt(); if (next >= 0) { nonnegatives++; } else { negatives++; } } public static void printEvenOdd(int max) { for (int i = 1; i <= max; i++) { if (i % 2 == 0) { System.out.println(i + " is even"); } else { System.out.println(i + " is odd"); } } }

• (

Many students new to if/else write code like this:

int percent = console.nextInt(); if (percent >= 90) { System.out.println("You got an A!"); } if (percent >= 80) { System.out.println("You got a B!"); } if (percent >= 70) { System.out.println("You got a C!"); } if (percent >= 60) { System.out.println("You got a D!"); } else { System.out.println("You got an F!"); }

What’s the problem?

9

• :if8else
• Nested if/else statement: A chain of if/else that can select

between many different outcomes based on several tests.

• General syntax:

if (<test>) { <statement(s)> ; } else if (<test>) { <statement(s)> ; } else { <statement(s)> ; }

• Example:

if (number > 0) { System.out.println("Positive"); } else if (number < 0) { System.out.println("Negative"); } else { System.out.println("Zero"); }

!

:if8else

A nested if/else can end with an if or an else.

If it ends with else, one of the branches must be taken. If it ends with if, the program might not execute any branch.

if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else { <statement(s)>; } if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; }

• :if8else 4 3

if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else { <statement(s)>; }

• :if8else if 4 3

if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; }

• :if8else

if (place == 1) { System.out.println("You win the gold medal!"); } else if (place == 2) { System.out.println("You win a silver medal!"); } else if (place == 3) { System.out.println("You earned a bronze medal."); }

Are there any cases where this code will not print a

message?

How could we modify it to print a message to non-

medalists?

• #, if 4 3

if (<test>) { <statement(s)>; } if (<test>) { <statement(s)>; } if (<test>) { <statement(s)>; }

10

• #-$if8else

Choose 0 or 1 set of statements if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } Choose 0, 1, or more set of statements if (<test>) { <statement(s)>; } if (<test>) { <statement(s)>; } if (<test>) { <statement(s)>; } Choose exactly 1 set of statements if (<test>) { <statement(s)>; } else if (<test>) { <statement(s)>; } else { <statement(s)>; }

• ;if8else 5

Reading the user's GPA and printing whether the student is on

the dean's list (3.8 to 4.0) or honor roll (3.5 to 3.8)

Printing whether a number is even or odd Printing whether a user is lower-class, middle-class, or upper-

class based on their income

Determining whether a number is divisible by 2, 3, and/or 5 Printing a user's grade of A, B, C, D, or F based on their

percentage in the course

• ;if8else 5

Reading the user's GPA and printing whether the student is on

the dean's list (3.8 to 4.0) or honor roll (3.5 to 3.8) if / else if

Printing whether a number is even or odd

if / else

Printing whether a user is lower-class, middle-class, or upper-

class based on their income if / else if / else

Determining whether a number is divisible by 2, 3, and/or 5

if / if / if

Printing a user's grade of A, B, C, D, or F based on their

percentage in the course if / else if / else if / else if / else

• 7if8else

int z; if (x > y) { z = x; } else { z = y; } double d = a; if (b < d) { d = b; } if (c < d) { d = c; } int z = Math.max(x, y); double d = Math.min(a, Math.min(b, c));

Just because you learned a new construct does not mean that

every new problem has to be solved using that construct!

• 0if/else

Readings: 4.3 (pg. 230 – 232)

!

0if/else

factoring: extracting common/redundant code Factoring if/else code reduces the size of the if and

else statements

Factoring tips:

• If the start of each branch is the same, move it before the if/else.
• If the end of each branch is the same, move it after the if/else.

11

• 0$*4

if (money < 500) { System.out.println("You have, $" + money + " left."); System.out.print("Caution! Bet carefully."); System.out.print("How much do you want to bet? "); bet = console.nextInt(); } else if (money < 1000) { System.out.println("You have, $" + money + " left."); System.out.print("Consider betting moderately."); System.out.print("How much do you want to bet? "); bet = console.nextInt(); } else { System.out.println("You have, $" + money + " left."); System.out.print("You may bet liberally."); System.out.print("How much do you want to bet? "); bet = console.nextInt(); }

• 0$"4

System.out.println("You have, $" + money + " left."); if (money < 500) { System.out.print("Caution! Bet carefully."); } else if (money < 1000) { System.out.print("Consider betting moderately."); } else { System.out.print("You may bet liberally."); } System.out.print("How much do you want to bet? "); bet = console.nextInt();

• # 4if8else

Readings: 4.3 (pg. 225 – 226)

• % <

String message; if (gpa >= 3.0) { message = "Welcome to the UW!"; } if (gpa >= 2.0) { message = "Have you considered applying to WSU?"; } if (gpa < 2.0) { message = "I hear Harvard still needs students..."; } System.out.println(message);

The compiler will complain that "variable message might not

have been initialized". Why?

• % <

The solution:

String message; if (gpa >= 3.0) { message = "Welcome to the UW!"; } else if (gpa >= 2.0) { message = "Have you considered applying to WSU?"; } else { // gpa < 2.0 message = "I hear Harvard still needs students..."; } System.out.println(message);

• 9

Methods can return different values under different conditions: public static int min(int a, int b) { if (a > b) { return b; } else { return a; } } public static String message(int place) { if (place == 1) { return "You won!"; } else { return "If you're not first, you're last!"; } }

12

• (

public static int min(int a, int b) { if (a > b) { return b; } }

The compiler will complain about a "missing return

statement". Why?

ERROR: Not returning a value in every path. In the above

example, what if a <= b?

• 235

public static int min(int a, int b) { if (a > b) { return b; } else if (a <= b) { return a; } }

It still produces the "missing return statement" error. Why?

To our eyes, it is clear that all paths (greater, equal, less) do

return a value.

But the compiler thinks that if/else if code might choose not

to execute any branch, so it refuses to accept this code.

How can we fix it?

• = $()

Write a method named countFactors that returns the

number of factors of a given integer.

For example, countFactors(60) returns 12 because 1, 2, 3,

4, 5, 6, 10, 12, 15, 20, 30, and 60 are factors of 60.

Write a method named numUnique that accepts two

integers as parameters and returns how many unique values were passed.

For example, numUnique(3, 7) returns 2 because 3 and 7 are

two unique numbers, but numUnique(4, 4) returns 1 because 4 and 4 only represent one unique number.

!

()$.

Write a program that prompts the user for a maximum

integer and prints out a list of all prime numbers up to that

• maximum. Here is an example log of execution:

Maximum number? 50 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 15 total primes

• # $.

import java.util.*; public class PrintPrimes { public static void main(String[] args) { Scanner console = new Scanner(System.in); printPrimes(getNumber(console)); } public static int countFactors(int num) { int count = 0; for (int i = 1; i <= num; i++) { if (num % i == 0) { count++; } } return count; } ...

• # $.

public static int getNumber(Scanner console) { System.out.print("Maximum number? "); return console.nextInt(); } public static void printPrimes(int max) { int numPrimes = 0; if (max >= 2) { System.out.print(2); numPrimes++; for (int i = 3; i <= max; i++) { if (countFactors(i) == 2) { numPrimes++; System.out.print(", " + i); } } System.out.println(); } System.out.println(numPrimes + " total primes"); } }

13

• >!

Readings: None

• ;-3?5

You arrive at your dorm after a thought-

provoking lecture of CSE 142. To feed your brain, you put some bread into your toaster

• ven and set the dial for 5 minutes. The

toaster oven ticks away. After five minutes, the toaster oven

• dings. You take the bread out,

but it’s not even toasted. What do you do?

• ;?335

import java.util.*; public class Buggy { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("How many numbers to average? "); int count = console.nextInt(); int sum = computeSum(console, count); System.out.println("The average is: " + (sum / count)); } public static int computeSum(Scanner input, int num) { int total = 0; for (int i = 1; i <= num; i++) { System.out.print("#" + i + ": "); total = input.nextInt(); } return total; } }

• " 3-

Learn how to use the debugger

See the notes on the web page under “jGRASP

Tutorial”

System.out.println is your friend. Use it

to print out variables and expressions.

Example:

System.out.println("x = " + x);